The subject matter of the invention relates to optical data storage in which information is recorded onto and retrieved from a record capable of storing the information in the form of optically imprintable markings and retrieving such information from these markings by optical devices.
Various forms of optical data storage are currently available for recording both digital and analog information For example, analog data is recorded on optical disks by encoding the data in the form of variable width pits or depressions in the record and then reading this information by electro-optical means.
Another example of existing optical data storage is provided by the popular "Compact Disc" technology in which music and other audio information is recorded onto small optical record discs that are capable of storing large amounts of prerecorded music and other audio tracks. This Compact Disc technology, made popular in the field of prerecorded music, is now being used to store computer data. Software programs and data can thus be stored on a physically small record in amounts many times greater than popular magnetic disc records. For example, a Compact Disc can store the equivalent amount of computer data that would require over one thousand "floppy" magnetic discs of the popular 51/4" size used in personal computers
Despite the impressive advances of optical data storage techniques, there are, nevertheless, limits to the amount of information that can be recorded on an optical record. Moreover, there are significant limitations in the low rate of data recording and data access available from existing optical storage techniques. Furthermore, advances are being made in magnetic recording technology and in other storage materials to increase their storage capacity, narrowing the net advantage of optical data storage systems.
As related background to the subject invention, the existing optical data storage techniques most typically store information in the form of a series of data bits in which each bit is represented on the record as a physical mark or transition from one physical mark to the next along the direction of scan. For example, data may be recorded as a series of pits spaced along a record track Each pit or transition between pit and surrounding land represents a single bit of data. In this prior type of optical storage, the data bit is encoded in the form of a physical mark on the record which can be read by optical means, such as by irradiating the object mark by a light source, usually from a semiconductor diode capable of emitting a small laser beam. Because each data bit requires a distinct mark, which we call an object mark, on the optical record, the amount of data that can be recorded is limited by existing means for forming the object mark and for sensing its presence or absence along the record track. In other words, each physical object mark on the record yields only one bit of data, and, of course, many hundreds of thousands of such data bits are required in order to store any significant amount of information.
Also, in regard to existing optical data storage systems, binary object marks on the record along the data track pose difficult practical problems of: tracking to insure proper alignment of the read/write optics with the data, focusing of the read/write optics to insure an adequate signal-to-noise ratio in the retrieved data, synchronizing the read electronics with recovered data bit signals, and compensation for the broad frequency band width of the detected data stream. These compensation or correction requirements for accurate tracking, focusing, synchronizing have led to the adoption of certain sophisticated bit/word encoding schemes, such as the well known 8 to 14 code. However, such encoding techniques reduce the amount of data that can be recorded over any given unit of length along the track, and limit the effective rate of data retrieval.
It is noted that some existing optical data storage systems use diffraction light patterns for tracking. However, it is important to distinguish the use of such diffraction light patterns for such a purpose, i.e., tracking, from the present invention's use of interference light patterns as described herein for actually encoding the basic information signal in a changeable interference pattern extracted from the record during read. Existing uses of diffraction patterns are limited to the formation of side lobe sensing windows located on opposite sides of a central read lobe and merely assist in maintaining the central read lobe on track center.
Also, the principle of the present invention is to be distinguished from certain holographic systems in which the optical interference pattern itself is recorded to enable reproduction of the holographic image.
By way of further background, reference is made to the following treatises dealing with optics and recording techniques relevant to an understanding of the present invention described herein:
General reference for single and multi-slit interference: Fundamentals of Optics, Jenkins and White, McGraw-Hill, 1950, Chapters 13, 15, 16, 17, and especially Sections 17.1, 17.2, and 17.3.
General reference for multi-element interference, the Fourier transform of them, and circular aperture formulas. Fourier Optics: An Introduction, E. G. Steward, Ellis Horwood Ltd., Publisher, Halsted Press/John Wiley & Sons, 1987, Chapters 2 and 4, and Appendix C.
CD ROM The New Papyrus, S. Lambert, S. Ropiequet, Eds., Microsoft Press, 1986.
Principles of Optical Disc Systems, G. Bouwhuis, J. Braat, A. Huijser, J. Pasman, G. van Rosmalen, K. Schouhamer Immink (all at Philips Research Laboratories, Eindhoven), Adam Hilger Ltd., 1985.